Update timezone/Makefile to use -Wno-unused-variable

    GCC 6.0 (prelease) complains about time_t_min and time_t_max
    not being used.  These variables are not used in glibc but
    are needed in other packages.

	* timezone/Makefile (CFLAGS-zic.c): Add -Wno-unused-variable.
	(CFLAGS-ialloc.c): Ditto.
	(CFLAGS-scheck.c): Ditto.